I'm having a similar issue when trying to access fields through relations.

The strange thing is that it works perfectly on a XAMPP server installed on OS X Lion, but throws a PHP notice (Trying to get property of non-object) on a WAMP servers installed on Windows 7. Any ideas, anyone?

Basically, the error was occuring when one of the values returned was null. Probably the PHP version running on OS X (I think it's 5.3.1) was not objecting, while PHP 5.3.9 on Windows was.

Hope this helps someone!

It helped. I had a few comments with the user_ids unset - filling them in fixed the issue (though obviously ensuring that the individual fields are not empty before displaying them is obviously the least risky solution.